Wires of the production of various types of springs, including valve springs, compression springs, torsion springs and extension springs are normally heat treated to produce desired physical properties such as tensile and/or compressive strength, toughness and ductility. In typical processes the wires, which may contain 0.4-8% C, 0.0-1% CR, 0-3%Va, 0-2% Si, balance Fe, are heated to an austenitizing temperature, i.e., a temperature at which part or all of the wire is converted to austenite. This normally requires that the wire be heated to a temperature of about 1,500 to 1,800.degree. F., depending upon the composition of the wire. The wire is then quenched in oil, water or other conventional quenching media to convert the material to martensite, then reheated and held at a temperature such as 600 to 1,200.degree. F. to temper the wire and obtain the desired physical properties.
Furnaces for the continuous heat treatment of steel spring wires have included muffle furnaces, in which the wires pass through pipes or ducts filled with a controlled atmosphere, salt baths, silica fluid beds, and molten lead bath. Unfortunately, due to poor heat transfer characteristics, most of these conventional furnaces, with the exception of molten lead, require prolonged heating times, typically from 2 to 10 minutes, to heat wire to its austenitizing temperature.
Inductive heating has been proposed as a method for reducing the time required to heat these wires. Inductive heating at low to moderate frequencies will not heat this size range of wires to their austenitizing temperature efficiently because steel becomes non-magnetic at its Curie temperature of about 1350.degree. F. The efficiency of conduction heating drops significantly past the wires Curie temperature and continued inductive heating requires large amounts of energy. However, wire can be heated to its austenitizing temperature more economically by heating the wire inductively to its Curie temperature and completing the heating process in a conventional furnace.
U.S. Pat. No. 4,788,394 to Vannese et al and U.S. Pat. No. 5,032,191 to Reiniche disclose processes for inductive heating of steel wire. In each, wires are passed through guides within the inductive heating chamber, with a coil surrounding all of the guides. Other proposed processes for inductive heating of wire also pass a number of wires through a common inductive heating coil. This has certain defects. The inductive heating system must be designed and adjusted for the bundle of wires as a whole, which complicates adjustment for any individual wire. If a wire is missing from the bundle, heating of the other wires can be affected adversely. For example, variations in the number or size of wire changes the conductive coupling properties of the coil, with resultant large and unpredictable variations in wire temperature.